Metal hardness is typically determined by indenting a metal specimen at a known force with a known device and measuring either the depth or diameter of the indentation. Diameter testing is most commonly referred to as Brinell testing. See ASTM E10 (10-78), incorporated herein by reference. In Brinell testing, a ball penetrator, either 5 mm or, more typically, 10 mm in diameter, is applied to a test specimen with a predetermined load to produce a generally spherical indentation in the specimen. Knowing the applied force, the Brinell hardness of the test specimen can be determined from the diameter of the indentation.
Current Brinell testing involves measuring the diameter of the indentation along orthogonal axes using either a small optical microscope with a graded reticle or an electro-optical probe device which indicates or responds to, respectively, the differences in light reflected from the specimen surrounding the indentation, the side walls of the indentation and the indentation itself. Each type of instrument can be positioned over and against a specimen. Each instrument generates a magnified image of the underlying specimen surface. The operator moves the instrument to try to center the instrument over the center of the indentation. Each instrument also typically employs light focused or otherwise directed through the instrument so that when the central axis of the instrument is perpendicular to the specimen surface, the light also strikes the specimen surface beneath the instrument to illuminate that area including the indentation.
A major problem with each type of instrument is the difficulty in locating and then accurately centering the instrument with respect to the indentation. In most instances, a field of view relatively large with respect to the size of the indentation must be provided in order to permit the operator to view a sufficiently large area of the specimen surface to orient the probe with respect to the indentation. In some instances, due to a roughness of the specimen surface and/or the polish of the side walls of the indentation, the contrast between light reflected from the undisturbed surface and the indentation is relatively low, making it difficult for the operator to optically identify the indentation and center of the indentation. While the newest optical Brinell hardness-measuring electrode probes are configured to compensate for misalignment of the probe with respect to the central axis of the indentation, such misalignment between the indentation and the central optical axis of the probe can still lead to some degradation of accuracy in the diameter measurement. In addition, because a relatively large field of view is provided to the optical system to permit the operator to find the indentation, the resolution of the optical system is reduced.
It would, therefore, be desirable to provide a means and a method to simplify alignment of a Brinell hardness optical measuring probe with a Brinell indentation which simplifies the measurement procedure in a way which does not adversely affect the measurement results.